1. Field of the Invention
This invention relates a process for recovering bitumen from a tar sand and, more particularly, to a hot-water recovery process specifically for Utah tar sands, the process utilizing a caustic wetting agent at a controlled pH range in the digester.
2. The Prior Art
The term "tar sand" refers to a consolidated mixture of bitumen (tar) and sand. Alternate names for tar sands are "oil sands" and "bituminous sands", the latter term being more technically correct in that the sense of the term provides an adequate description of the mixture. The sand in tar sand is mostly alpha quartz as determined from X-ray diffraction patterns. The bitumen or tar of the tar sands consists of a mixture of a variety of hydrocarbons and, if properly separated from the sand component, may be used as feedstock for the production of synthetic fuels and/or petrochemicals.
Tar sand deposits occur throughout the world, often in the same geographical areas as petroleum deposits. Significantly large tar sand deposits have been identified and mapped in Canada, Venezuela, and the United States. The Canadian tar sand deposits are known as the Athabasca tar sands and are located in the province of Alberta, Canada. The estimated reserves for the bitumen content in the Athabasca tar sands has been estimated to be approximately 900 billion barrels and, to date, is the only tar sand deposit in the world currently being mined and processed for the recovery of the bitumen content.
Analysis of the Athabasca tar sands indicates an average bitumen content of approximately 12-13% by weight. Importantly, the Athabasca tar sands also have a relatively high moisture content of approximately 3-5%, by weight, connate water. It has, therefore, been postulated by certain investigators that the equilibrium structure of the Athabasca tar sands consists of a sand mixed with but separated from the bitumen matrix by a film of connate water, the connate water surrounding each grain of sand. Accordingly, it is further postulated that the bitumen in the Athabasca tar sands has already been displaced from the sand grains by the connate water. Under these conditions, bitumen separation involves a relatively simple phase disengagement process whereby the bitumen phase is readily disengaged from the sand phase by the conventional hot water separation techniques.
A more comprehensive discussion of the Athabasca tar sands may be found in the literature including, for example, (1) E. D. Innes and J. V. D. Tear, "Canada's First Commercial Tar Sand Development," Proceedings of the Seventh World Petroleum Congress, Elsevier Publishing Co., 3, p. 633, (1967); (2) F. W. Camp, The Tar Sands of Alberta Canada, Second Edition, Cameron Engineering, Inc., Denver, Colorado (1974); and (3) J. Leja and C. W. Bowman, "Application of Thermodynamics to the Athabasca Tar Sands," Canadian Journal of Chemical Engineering, 46, p. 479 (1968).
Additionally, the following U.S. Patents are a few of the patents which have been granted for apparatus for processes directed toward obtaining bitumen from tar sands: U.S. Pat. Nos. 1,497,607; 1,514,113; 2,871,180; 2,927,007; 2,965,557; 3,161,581; 3,392,105; 3,553,099; 3,560,371; 3,556,980; 3,605,975; 3,784,464; 3,847,789; 3,875,046; and 3,893,907. With the exception of U.S. Pat. No. 3,605,975, each of the foregoing patents have been directed toward processing Athabasca tar sands and are believed, therefore, not directly applicable to processing Utah tar sands as will be set forth more fully hereinafter.
According to a report by the Utah Geological and Mineral Servey, the State of Utah contains at least 25 billion barrels of bitumen in the form of Utah tar sands. This represents approximately 95% of the total mapped tar sand reserves in the United States. Although the Utah tar sand reserves appear small in comparison with the enormous potential of the Athabasca tar sands, the Utah tar sand reserves represent a significant energy resource when compared to the United States crude oil proven reserves (approximately 31.3 billion barrels) and the United States crude oil production of almost 3.0 billion barrels during 1976. Utah tar sands occur in six major deposits along the eastern edge of the state and the bitumen content varies from deposit to deposit as well as within a given deposit. However, the current information available indicates that the Utah tar sand deposits average generally less than 10% bitumen by weight but have been found with a bitumen saturation of up to 17% by weight.
Importantly, unlike the Athabasca tar sands, Utah tar sands have been found to be so dry that no moisture content can be detected by standard analytical techniques. Accordingly, it is obvious that in the absence of connate water, the bitumen is directly in contact with and bonded to, the surface of the sand grains. In addition, tests have also determined that bitumen obtained from Utah tar sands is two orders of magnitude or about 100 times more viscous than bitumen obtained from Athabasca tar sands. Hence, processing of Utah tar sands must involve both bitumen displacement of the bonded bitumen from the sand grains and subsequent phase disengagement of the more viscous bitumen from the sand phase.
Attempts have been made to process Utah tar sands with the hot water processes used for the Athabasca tar sands. However, these attempts have been unsuccessful in light of the recognized differences in the physical and chemical nature of the Utah tar sands, as compared to the Athabasca tar sands. It would, therefore, be an advancement in the art to provide an improved process for recovering bitumen from Utah tar sands or, more particularly, from a tar sand which is characterized by the substantial absence of connate water. It would be an even further advancement in the art to provide a hot-water process for recovery of bitumen from dry tar sands. Such a process is disclosed and claimed in the present invention.